BEIRUT — Neighborhoods across Damascus were rocked Monday with the most intense fighting the capital has seen in the 16-month-old Syrian uprising, as Russia faced strong diplomatic pressure to sign on to a U.N. resolution that would impose strict sanctions on the Assad government.

The escalation in fighting in Damascus, along with the possibility of increased economic pressure, could turn out to be the most serious challenge yet forthe regime of President Bashar al-Assad.

Deepening the isolation of the Assad regime, Morocco ordered Syria’s ambassador to leave the North African kingdom and called for a transition to democracy in Syria, the Reuters news agency reported. The Assad government retaliated by ordering the expulsion of Morocco’s ambassador to Syria.

But Russia has remained steadfast in its refusal to abandon its alliance with the government in Damascus.

A Russian ship carrying repaired helicopters — which had set sail for Syria under a Curacao flag but turned back when it lost its insurance — is expected to sail to Syria from St. Petersburg, according to the Interfax news agency. Russia says it is not arming Assad but is complying with existing maintenance contracts.

At the United Nations, American and European diplomats on Monday abandoned an effort to adopt a Security Council statement condemning Syria’s “outrageous use of force” last week in the western village of Tremseh, citing Russia’s decision late Friday to block the statement.

The Security Council remained deeply divided, with the United States and its European allies favoring the adoption of a resolution threatening sanctions against Damascus if it failed to halt its shelling of residential areas. Russia’s vehement opposition to sanctions has raised concerns that it may be preparing its third veto of a Western-backed resolution aimed at punishing Assad for his excessive use of force in civilian areas.

Russia has circulated a draft resolution that would urge the Syrian government and theopposition to reach agreement through talks but would not include any measures compelling them to do so.

Foreign Minister Sergei Lavrov, speaking at a news conference in Moscow, said Assad will not be forced from power — not because Russia protects him, but because he still has strong support within Syria. “We have been hearing statements that a key to the Syrian settlement is in Moscow,” he said. “They tell us that we should persuade Assad to step down of his own free will. That is simply not realistic.”

Lavrov met later with Kofi Annan, the U.N. special envoy who has been trying to work out a peace plan for Syria — an effort that Russia says it backs. Annan plans to meet with Russian President Vladimir Putin on Tuesday.

Meanwhile, U.N. Secretary General Ban Ki-moon was headed to Beijing Tuesday to press Chinese leaders to back a stronger Security Council response to Syria. Ban will speak with President Hu Jintao on Wednesday.

The fighting in Damascus on Sunday and Monday was concentrated primarily in southern neighborhoods, including Tadamon, Kfar Sousa, Nahr Aisha and Maidan. But residents reached by telephone said it appeared to be moving closer to the city center. The Local Coordination Committees, an opposition group, reported a series of explosions Monday in Yarmouk, a neighborhood where most residents are Palestinian refugees.

The official Syrian Arab News Agency mentioned the Damascus clashes in a news item, saying that weapons, ammunition and cars were confiscated and that “scores of terrorists were killed.”

The Syrian military continued shelling the neighborhoods Monday and Tuesday as street clashes with rebel fighters ramped up, according to opposition activists, who said government forces used helicopter gunships to fight rebels in Damascus. The highway to Damascus airport was briefly blocked by the military Monday, according to residents, and armored vehicles were positioned in areas of heavy fighting.

“I’ve heard bombings nonstop since yesterday,” Rama, a 25-year-old Damascus resident who asked that her real name not be used because she feared for her safety, said in a Facebook chat. “Damascus is divided now into neighborhoods with gunfire and shelling and safe neighborhoods where you only hear the sounds.”

Monday’s fighting in the capital resulted in the death of 10 civilians and opposition supporters and eight soldiers, according to the Britain-based Syrian Observatory for Human Rights.

An opposition video aired on al-Jazeera showed rebel fighters firing machine guns and rocket-propelled grenades while taking cover behind sandbag shelters. One particularly gruesome video showed the blackened corpses of alleged members of the pro-government shabiha militia burning inside a civilian car after a rebel attack in Maidan. Activists in other parts of the city tried to show their support for the rebels by staging demonstrations and blocking roads with flaming debris, according to reports by opposition groups.

Lena Shami, an activist in Damascus who asked that her real name not be used to protect her safety, said the clashes were going to increase in the capital. “We are now expecting more fighting inside Damascus and more escalation,” she said by telephone, as the sound of gunfire and explosions could be heard in the background.

The United Nations’ humanitarian relief coordinator, Valerie Amos, warned the Syrian government and the armed opposition Monday that the rules of war now apply in Syria and that the direct targeting of civilians could bring war-crimes charges. The remarks follow a determination Sunday by the International Committee of the Red Cross that the fighting in parts of Syria constitutes an internal armed conflict, subjecting combatants to the Geneva Conventions.

“I call on all parties to the conflict to uphold their obligation to distinguish between civilians and combatants and to take all feasible precautions to avoid civilian injury and loss of life,” she said in a statement. “Intentionally directing attacks against civilians constitutes a war crime.”

In a sign of the diplomatic tensions, Russia’s U.N. ambassador, Vitaly I. Churkin, stormed out of a closed-door Security Council meeting Monday afternoon on Syria, saying there was no need to discuss the Western-backed resolution threatening sanctions, according to three council diplomats who were present.

The Russian walkout followed a sharp exchange between Churkin and his British counterpart, Mark Lyall Grant, who opened a discussion on the elements of the Western draft that would give Syria 10 days to withdraw its heavy weapons from Syrian towns or face sanctions. The British envoy said that although there was no agreement on the sanctions provision, the council should begin discussing other, less contentious, provisions in the text.

Churkin interrupted Lyall Grant, saying he “didn’t see the point in negotiating on your draft” if it included a Western-backed provision – under Chapter VII of the U.N. Charter — threatening sanctions, according to one diplomat. “You’re welcome to stay or leave,” Lyall Grant responded, according to a second diplomat who heard the exchange.

Churkin walked out of the consultations, retreating to an adjoining waiting room where he watched CNN. His deputy, Alexander Pankin, stayed behind. Churkin later returned to the council deliberations.

Englund reported from Moscow. Colum Lynch at the United Nations and Suzan Haidamous and a special correspondent in Beirut contributed to this report.

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